Offshore platform having a partially removable drilling deck



Aug. 25, 1970 LLOYD ET AL 3,525,392

OFFSHORE PLATFORM HAVING A PARTIALLY REMOVABLE DRILLING DECK Filed Dec. 10, 1968 25 2&2 A

WAWAY Fl 6'. I

JAMES R. LLOYD INVENTORS CHARLES E. KOLODZEY KENT T. BAILEY ATTORNEY I United States Patent "ice 3,525,392 OFFSHORE PLATFORM HAVING A PARTIALLY REMOVABLE DRILLING DECK James R. Lloyd, Houston, Tex., Charles E. Kolodzey, New Orleans, La., and Kent T. Bailey, Houston, Tex., assiguors to Esso Production Research Company, a corporation of Delaware Filed Dec. 10, 1968, Ser. No. 782,574 Int. Cl. E21b 7/12, 43/01 US. Cl. 166.5 6 Claims ABSTRACT OF THE DISCLOSURE Field of the invention This invention relates to offshore platforms useful in drilling for and producing crude oil and natural gas. More particularly, the invention is concerned with bottom-supported offshore drilling and production platforms that have partially removable drilling decks.

Description of the prior art The search for crude oil and natural gas has been rapidly extended into deeper waters in recent years. Prior to 1958 the deepest water in which production operations had been conducted was 90 feet. Today, wells are being produced in water 340 feet deep and preparations are being made to produce oil and gas from wells located in more than 650 feet of water. Experience has shown that conventional bottom-founded platforms having the production equipment mounted above the ocean surface may not be satisfactory for such operations. The forces and moments imposed on these plaftorms by waves and currents in deep water substantially increase the risk of platform failure during severe storms. Because of this increased risk and the generally increased platform size, the erection of safe platforms in deep water is exceedingly expensive.

One approach suggested for deep water is the use of a submerged platform that extends to within diver depth of the surface. The investment required to construct such a platform is substantially less than that for a platform that extends to the surface. One reason for this is the smaller size of such platforms. Another factors is that the stresses in structual members due to surface wave action are generally lower. The objection to platforms of this type is that they are difficult to install and require that drilling and maintenance operations be conducted for the most part from expensive floating vessels.

Another approach that has been suggested is to drill the wells from a platform that extends to the surface and that has a removable top. The wells are completed on a submerged deck, and the upper portion of the platform is removed after drilling operations are completed, leaving only a submerged platform with wellheads mounted at diver depth. The advantage of this approach, compared with a fully submerged platform, is the saving in drilling costs over outlays required for floating drilling. One disadvantage is that all maintenance operations must be performed from an expansive floating platform or by divers operating from a work boat. Another difficulty is that the production equipment must either be mounted on a floating production facility or located undersea, both of 3,525,392 Patented Aug. 25, 1970 which are expensive. These disadvantages may limit the use of such platforms.

SUMMARY OF THE INVENTION The platform of this invention alleviates many of the problems inherent in earlier suggested deepwater platforms. This platform is a bottom-supported drilling and production platform having a driling deck mounted above the water surface and a submerged well support deck on which the wellheads are mounted at diver depth. The upper section is partially removable so that a substantial portion of the drilling deck can be removed after drilling operations are completed, leaving only a small production deck protruding above the water surface. Production equipment is mounted on this small deck and flowlines from each of the submerged wells are connected to it directly or through manifolding so that the wells are accessible from the production deck for maintenance with pumpdown tools.

The platform of the invention exhibits far'more transparency to wind, waves and currents than platforms that have a full deck above the water surface because only a small production deck protrudes above the water surface. This increased transparency reduces the forces and moments acting on the platform, and thus reduces the risk of structural failure during severe storms. The removable drilling deck can be used again on subsequent platforms, thereby reducing platform investment. Drilling costs are substantially less than those normally incurred with wholly submerged platforms because the drilling equipment is supported on the drilling deck of the structure, rather than on a floating drilling vessel. Savings in well maintenance costs over those wtih platforms having fully removable drilling decks accrue because the small production deck is in'comrnunication with the wellheads, thereby enabling routine well maintenance operations to be performed from the production deck. A floating vessel is required only when major workover operations are undertaken. The small production deck also can be employed to support diver operations since life support apparatus, communication equipment, and hoisting equipment normally placed on the divers workboat can be mounted on the production platform. This may eleminate the need for a workboat, further reducing well maintenance costs. In addition, the production deck eliminates the need for a floating producing facility or for submerged production equipment and thus lowers the required investment still further. It should therefore be apparent that the drilling and production platform of the invention has significant advantages over structures suggested heretofore.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front view of a platform constructed in accordance with the invention in which a production deck is mounted on a single leg structural extension.

FIG. 2 is a left side. view of the platform of FIG. 1.

FIG. 3 is a front view of the upper portion of a platform constructed in accordance with the invention in which a production deck is mounted on a double leg structural extension.

FIG. 4 is a left side view of the platform of FIG. 3.

FIG. 5 is a top view of the platform depicted in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 depicts an offshore drilling and production platform constructed in accordance with the invention. The frame is constructed of elongated tubular legs 13 and a number of horizontal and diagonal braces, 15 and 17, respectively. The frame is, of course, not limited to a four-sided structure, to tubular members, or to the truss configuration shown. The invention is applicable to any style of bottom-founded platform. Buoyant caisson members form the lower portion of the platform legs and increase structural stability while the platform is being submerged during installation. Long caissons 19 serve as the legs that are to be submerged first; while short outrigger caissons 21 form part of the other two legs of the platform. The placement and relative size of the caissons, while suitable for the structure depicted, will be somewhat altered for platforms having a ditferent configuration, e.g., a different number of legs.

FIG. 1 illustrates the platform as it appears after installation. Means for anchoring the lower section of the structure to a submerged bottom may include piles 23 which are shown extending from the caissons into the submerged bottom 25. The piles may be driven or drilled, and although depicted as having been cut off near the lower end of the caissons can extend substantially higher. The number and size of piles necessary to anchor the platform can be determined in a conventional manner and will vary with size and height of the structure, oceanographic conditions and composition of the bottom. Grouting material can be used to bond the caissons and pilings to each other and to the submerged bottom to complete the anchoring of the structure.

A submerged wellhead support deck 27 is depicted in FIGS. 1 and 2 supported by caissons 19 and by horizontal brace members 15. Although only a partial deck is shown mounted on the submerged lower section of the platform, the support deck may instead be a full deck. This submerged deck supports wellheads 31, shown in FIG. 2, which are mounted on the upper end of the conductors 29. Placement of the wellheads on the support deck makes access convenient for divers who can freely move around this submerged deck. Each of the wellheads has two upwardly extending tubing extensions so that pumpdown maintenance tools can be introduced into the wells. A conventional radius loop may be installed in each of the tubing extensions at the wellhead to permit the direction of the extensions to be abruptly changed without impeding the passage of pumpdown tools. The tubing extensions 33 are then passed directly or through manifolding to the base of leg extension 35 and are then extended upwardly to the production deck 37 where they are accessible for maintenance operations.

The permanent production deck 37 is shown supported in FIGS. 1 and 2 by a single tubular leg extension 35. Other structural configurations for this fixed upper section of the platform are suitable, so long as the surface area of the support member acted upon by wind, waves, and current is not unduly large. The production deck may support separation equipment, pumps, compressors, and associated equipment required to produce and process crude oil and natural gas and introduce them into pipelines. It may also house controls to operate subsea valves and other submerged equipment, equipment necessary for pumpdown maintenance operations, a life support system for divers, and a crane or similar device to help in lifting heavy equipment. Although the production deck is preferably mounted above the wellhead support deck for ease of access to the wellheads, it can conveniently be located at other points above the submerged platform.

The removable upper section is shown by dashed lines in FIGS. 1 and 2 and includes three additional leg extensions 39 for providing support. The removable drilling deck 41 abuts against production deck 37 and is locked to it with heavy pins or other conventional means during drilling operations. Additional support for the drilling deck is provided by diagonal and vertical braces, 43 and 45. The drilling rig, and associated support equipment, not shown, are mounted on top of the drilling deck. Many other configurations of removable drilling decks would be equally suitable. The removable deck could, for example, be mounted well above the permanent production deck.

An alternate configuration of the platform of the invention is shown in FIGS. 3, 4 and 5. The numbering of structural features corresponds With that of FIGS. 1 and 2. The distinguishing feature of the alternate structure is best depicted in FIG. 4 in which it will be noted that the production deck 37 is larger than that of the structure of FIGS. 1 and 2 and is supported by two leg extensions 35. The permanent production deck is positioned directly above the completed Wells on the wellhead support deck below as shown in FIG. 5. This facilitates the use of lifting equipment and the like when it is desired to perform maintenance operations on the wellheads or on other equipment mounted on the wellhead support deck.

Prior to installation, the drilling and production platform of the invention may be floated to the drilling site with or without the drilling deck in place and with the long caissons 19 in the lowermost position. If the tubular members and caissons do not add enough buoyancy for the structure to float, supplementary floats can be attached to increase the buoyancy. Once the drilling site is reached, the lower section of the structure is submerged by controlled flooding and the platform assumes a vertical orientation. Then, with the caissons resting on the submerged bottom 25 and the structure in an upright position, the pilings are set. A derrick barge or similar work boat may be used to drill or drive the piles down through the caissons into the submerged bottom. Where bottom conditions permit, the drilling deck and drilling equipment may be installed first and the piles can then be drilled. The piles are grouted or welded to the caissons and cut off to complete the anchoring of the structure to the submerged bottom.

Once the platform has been anchored in place, the drilling deck 41 and drilling equipment may be mounted on the removable upper section of the platform and drilling operations initiated. The conductors are first lowered through lateral supports and driven, washed down or drilled into the submerged bottom. Although the preceding procedure is preferred, the piles can also be used as conductor pipes. When they are, however, wellhead space is somewhat limited. The upper end of each conductor pipe is terminated on the wellhead support deck and a wellhead is mounted on this upper end. A drilling riser is coupled to the conductor pipe at the Wellhead and the two are used to maintain mud circulation back to the surface and therefore the riser must be extended to a point above the water surface. After a well has been drilled and completed, the drilling riser is removed.

After drilling operations are complete, or if desired, commensurate with drilling operations, flowlines, control lines, and associated apparatus are connected from the submerged wellhead support deck up to that portion of the platform that will become the permanent production deck. When drilling operations are complete, a derrick barge may be pulled alongside to remove the drilling equipment. The temporary drilling deck section is disassembled and removed, leaving only the permanent production deck. Production equipment is then installed on the permanent production deck. A pipeline riser, not shown, may be extended up caisson 19 and leg extension 35 to the production deck to facilitate transfer of produced oil or gas into a pipeline to market. If construction of a pipeline is impractical, floating production storage can be moored nearby or a submerged storage tank can be installed on the bottom. A flexible line may be used to connect the production deck of the platform to the storage vessel or submerged storage.

What is claimed is:

1. A drilling and production platform comprising:

(a) a submersible lower section;

(b) means for anchoring said lower section in contact with a submerged bottom;

(c) a fixed deck supported by said lower section and extending at least partially across the lower section for supporting submerged wellheads at a diver accessible depth;

(d) a fixed upper section attached to said lower section for supporting production equipment above the water surface; and

(e) a removable upper section mounted on the lower section for supporting drilling equipment above the water surface.

2. A drilling and production platform as defined in claim 1 wherein said anchoring means includes a plurality of piles that extend from the lower section into the submerged bottom.

3. A drilling and production platform as defined in claim 2 including a plurality of conductor pipes extending downwardly from said fixed deck into the submerged bottom and laterally supported by the lower section.

4. A drilling and production platform as defined in claim 3 wherein said fixed upper section and said removable upper section abut against one another and the upper ends of the two sections form a single, level deck suitable for supporting drilling equipment.

5. A drilling and production platform as defined in claim 3 wherein said removable upper section extends above the fixed upper section.

6. A drilling and production platform as defined in claim 5 wherein conduits extend from wellheads mounted on the fixed deck to the top of the fixed upper section to permit the introduction of pumpdown tools into the wellheads from the fixed upper section.

References Cited UNITED STATES PATENTS 3,125,171 3/1964 Stewart 1757 X 3,379,245 4/1968 Manning 1665 3,412,564 11/1968 McClintock 6146.5

ERNEST R. PURSER, Primary Examiner U.S. Cl. X.R. 

